Selecting Fault Current for ETAP: Sub-Transient or Sync

[proznick]

I am doing a ground grid study for a small islanded hydro facillity connected to a utillity diesel plant. Using ETAP to model the short circuit ground currents in the hydro plant substation (step and touch potentials).

In setting up the model for the utillity diesels, should the sub-transient current fault values provided by the utillity be used or the synchronous fault current values.
In designing ground grid for sub, worst case would be using sub-transient values.

When modeling the ground grid in ETAP, it wants "Fault Current" and the "X/R" ratio which makes me think that it wants the synchronous fault current and will compute the maximum sub-transient current level using th X/R ratio. If this is the case, when using the subtransient current value instead, will make for overdesigned ground grid size.

Any thoughts on this appreciated.

cheers,

RP

 

[dpc]

Nope - you need to use the sub-transient reactance when determining step and touch potentials. If it uses the X/R at all it will be to determine asymmetrical current from the symmetrical current. The generator reactances have to do with the response of the rotating machine to a fault condition and are not directly related to the system X/R ratio.

The synchronous reactance of a generator is usually over 100%, meaning the fault current at the synchronous reactance is less than the full load current of the generator. This assumes a constant field, which is generally not a good assumption.

But if your local diesels are at the same voltage level as your substation, you need to take into account the generator grounding method. It may be resistance grounded.